1. Field of the Invention
The present invention relates to a recording head provided with a function for optically or thermally exciting a magnetic recording medium, locally reducing the coercivity of the recording medium and thermo-magnetically recording information and an information recording apparatus using it.
2. Description of the Related Art
In Japanese published unexamined patent application No. Hei10-162444, “Magneto-optical recording medium, recording/reading method and recording/reading apparatus” (first prior art), technique utilizing a head using a solid immersion lens in a recording/reading apparatus for recording a hyperfine magnetic domain signal at a hyperfine light beam spot on a magneto-optical disk is disclosed.
Besides, in Japanese published unexamined patent application No. 2001-255254, “Optical near field probe, optical near field optical microscope using it and optical recording/reading apparatus” (second prior art), technique related to an optical near field probe formed by a metal diffuser in a shape of a cone or a triangle formed on a substrate and a metallic, dielectric or semiconductor film formed around the diffuser and having the same thickness as the height of the diffuser and optical recording using it is disclosed.
Further, in Japanese published unexamined patent application No. 2002-48697, “Optical near field probe and its manufacturing method” (third prior art), a method of manufacturing a flat optical near field probe in which a metal diffuser is buried in a dielectric and the mean surface roughness of which is approximately 10 nm or less is disclosed.
In a recording head provided with a function for optically or thermally exciting a recording medium to change a local physical state of the recording medium and recording information and an information recording apparatus using it, a method of using a light spot stopped down up to a diffraction limit using a lens is general. In this case, the size of the light spot is approximately λ/NA when the wavelength of a light source is λ and the numerical aperture of the lens is NA and the magnitude determines the size of an area (a so-called recording mark) in which a physical state is changed on the recording medium, that is, recording density. To increase recording density, directly the wavelength of the light source is reduced or the numerical aperture is widened, however, the development of a light source having a shorter wavelength has an extremely great problem in the material of a light source element and the efficiency of emission and the numerical aperture cannot be theoretically made larger than 1. Therefore, in information recording depending upon the light spot at the diffraction limit, there is great difficulty to increase recording density.
To conquer the difficulty, thermomagnetic information recording technique using an optical near field disclosed in the first prior art is developed. That is, the diameter of the light spot used in recording is reduced from λ/NA further to 1/n or 1/n^2 (^: power) by using the solid immersion lens made of material the refractive index of which is n so as to increase recording density. However, a high refractive index and low absorptance in a wavelength of a combined light source are required for material forming the solid immersion lens. In case a violet semiconductor laser having the shortest wavelength currently realized as a light source for recording for example is used, λ is approximately 405 nm, however, the refractive index of optical glass that hardly absorbs in this waveband is approximately 2 and even if the optical glass is combined with a lens NA of which is 0.9, the diameter of the light spot is reduced only to approximately 110 nm. Therefore, it is difficult to stably form a recording mark in size by far smaller than the diameter of the light spot and the solid immersion lens has a limit in increasing recording density.
To conquer the limit in the first prior art, technique for generating an optical near field using a metal diffuser disclosed in the second and third prior arts is developed. As the distribution size of an optical near field equivalent to the diameter of a light spot is determined by the manufacuring precision of the metal diffuser in the prior arts, the distribution size of an optical near field can be greatly reduced by using high-resolution lithography depending upon an electron beam lithography, compared with the diameter of a light spot by the solid immersion lens. However, in these prior arts, the metal diffuser that generates an optical near field is exposed at the bottom of the recording slider and has structure that the metal diffuser is in contact with only a dielectric the thermal conductivity of which is relatively low. Therefore, a problem that when a part of recording energy is absorbed by the metal diffuser, the temperature rapidly rises, the metal diffuser is gradually oxidized, a function as the diffuser is damaged and in the worst case, the diffuser is dissolved and broken exists. Therefore, it is required that incident light from the light source can be converted to an optical near field at higher efficiency. Besides generally, as a dielectric and metal forming the diffuser are greatly different in a coefficient of thermal expansion, great stress is caused on an interface between both, and as possibility that the metal diffuser may peel and fall from the dielectric is high, the realization of a reliable recording head and a reliable information recording apparatus is hindered.
Besides, in case the similar thermomagnetic recording to a current magneto-optical disk is applied to a perpendicular magnetic recording medium advantageous in high-density magnetic recording using the second and third prior arts, the transition width of magnetization in the magnetic recording layer is determined by only a temperature gradient on the recording medium in recording. Therefore, unless the distribution size of an optical near field is reduced, the transition width of magnetization cannot be reduced and there is a limit to increase recording density without increasing the manufacuring precision of the metal diffuser.